1. Field of the Invention
This invention relates to an output circuit, and more particularly to an improvement of an output circuit used for a pulse driven type hearing aid which applies a predetermined pulse to the earphone of the hearing aid to drive an earphone, for example.
2. Description of the Related Art
It has been proposed in the past in pulse driven type hearing aids that an earphone is driven by the pulse signal after an audio signal which is input from a microphone is modulated by the pulse width modulation system (PWM) into a pulse signal, the frequency of which is higher than the frequency band of the audio signal.
In this type of hearing aid, the distortion of a signal to be generated when an audio signal is amplified is reduced so that sound with less distortion can be obtained from the earphone.
In addition, in this type of hearing aid, the output circuit can be constituted by a switching circuit using transistors. In this way, the circuit configuration of the output circuit can be simplified and at the same time the output circuit can be operated at a low power supply voltage.
A typical configuration of this type of hearing aid is shown in "An output amplifier whose time has come," HEARING INSTRUMENTS, VOLUME 39, NO. 10, 1988.
That is, in this hearing aid, as shown in FIG. 1, an audio input signal S1 obtained by a microphone (not shown) is input into an output circuit 2 through an input terminal 1.
The output circuit 2 modulates the audio input signal S1 into a pulse signal S2, the duty ratio of which is proportional to the amplitude at each point of the above audio input signal S1 (hereafter referred to as "instantaneous amplitude"), supplies the pulse signal S2 to an earphone 3, and drives it. In this way, an audio output signal S3 can be obtained.
That is, the output circuit 2 forms a pulse signal S2 such as shown in FIG. 2B and outputs it to the earphone 3 in accordance with the instantaneous amplitude of this audio input signal S1 when an audio input signal S1, such as shown in FIG. 2A, is input. The earphone 3 emits the audio output signal S3 such as shown in FIG. 2C into the external auditory canal in accordance with this pulse signal S2.
However, in this output circuit 2, as shown in FIGS. 2A and 2B, in cases where the audio input signal S1 is not input (that is, even in cases where the instantaneous amplitude of the audio input signal S1 is zero level), the pulse signal S2, having the duty ratio of 50%, is still generated. When this pulse signal S2 is applied to the earphone 3, the output circuit 2 consumes the same power as when the amplitude is large, even when the amplitude is small. This type of output circuit was insufficient in the point of power consumption.
As a method of solving such a problem, it is possible to consider the reduction of power consumption by separating the positive portion of the amplitude of the audio input signal from the negative portion by providing a rectifier circuit, applying a positive pulse to the positive portion and a negative pulse to the negative portion, and not applying a pulse at no-signal.
However, there is a problem in this method that the tone quality is greatly deteriorated because crossover distortion is generated by the voltage offset of a rectifier circuit, etc., especially when the signal level of the input audio signal is low. It is therefore necessary to form a complicated circuit configuration or provide an adjustment circuit to avoid that. As a result, the circuit scale becomes larger and the manufacturing cost increases. Therefore, it is difficult to use, especially, in the small equipment as a hearing aid.